Use of gel-based compositions for reducing the production of water in oil- or gas-producing wells

ABSTRACT

The invention has as its object the use of new composition of gels that can be used for the reduction of the production of water in oil- or gas-producing wells. 
     The compositions of gels comprise a solution of at least one water-soluble complex of a polyvalent metallic cation such as zirconium lactate able to cross link a nonionic polysaccharide such as scleroglucan, at a concentration of 2 to 10,000 parts per million of parts, expressed by weight of metal dioxide. With a preferred concentration of 2 to 100 ppm, the oil permeability is not appreciably affected. 
     The compositions make possible the formation of aqueous gels of scleroglucan that can be used for the prevention of inflows of water into the hydrocarbon-producing wells.

BACKGROUND OF THE INVENTION

This invention has as its object the use of new crosslinkingcompositions for polysaccharides, in particular scleroglucan. It alsorelates to the use of aqueous gels including these crosslinkingcompositions for the selective reduction of the production of water inoil- or gas-producing wells. They exhibit a particularly great advantagewhen the permeability of the formation treated in the vicinity of thewells is high and/or when the water produced is hot and/or salty, forexample at a formation temperature of 70° to 130° C. and/or a salinityof the water produced at least equal to that of sea water (at least 30g/l, expressed in NaCl). These new compositions are particularlyapplicable to the enhanced recovery of hydrocarbons.

The recovery of the liquid or gaseous hydrocarbons of subterraneanformations is very frequently accompanied by the production of largequantities of water. In some cases, although significant productions ofhydrocarbons are obtained, the production of water is so considerableand the costs for treating the water so high that the production ofhydrocarbons is not economical. In heterogeneous reservoirs, theexcessive production of water is often caused by the fingering of thewater injected through the zones of high permeability. This leads to apremature breakthrough of the water to the production well, to a poorvolumetric sweep and finally to an ineffective recovery of thehydrocarbons.

Many methods intended to reduce the production of water of the verypermeable formations have been proposed and tested in the field; theygenerally involve introducing into the formation, at the level of thezone to be isolated, either a cement or a suspension of solid particlesor of paraffins. Resins or gels of water-soluble polymers have morerecently been proposed and put into use. All these processes exhibit thedisadvantage of not being selective and of blocking the circulation ofthe oil or gas almost as much as that of the water.

More recently, the use of water-soluble polymers of high molecularweight in the absence of any crosslinking or coupling agent has beenproposed, polymers which exhibit the advantage, relative to thepreceding solutions and in particular those which use resins or gels ofpolymers, of reducing the circulation of the water without affecting theproduction of oil or gas in a troublesome way.

Among these water-soluble polymers, nonionic polysaccharides and inparticular scleroglucan have proved particularly effective in reducingselectively the production of water of production wells whilemaintaining the production of hydrocarbons. Thus, U.S. Pat. No.4,718,491 and patent application FR 89/1716 of the applicant recommendthe use of different polysaccharides, and in particular of ascleroglucan, in the absence of any crosslinking or coupling additive,for the selective reduction of the permeability in the vicinity of anoil- or gas-producing well. If the preferred range of application ofthese polymers covers the productions of hot (up to 130° C.) and Saltywater, its effectiveness decreases when the permeability of theformation becomes high and in particular if it is greater than 1 Darcy.

French patent application 90/13385 describes a crosslinking compositioncomprising at least one water-soluble complex having a base ofpolyvalent metallic cation and of complexing organic acid of the cationable to crosslink a polysaccharide, in particular scleroglucan.

U.S. Pat. No. 4,647,312 furthermore recommends the use of scleroglucancomplexes and a polyvalent metal ion such as titanium, zirconium andchromium for the production of fluids of very high viscosity and theiruse in enhanced recovery of oil. Although no mention is made in thispatent of the ability of these gels to propagate in a subterraneanformation, the increase in viscosity obtained shows that it is stronggels which should not have any selective character and therefore reducethe production of water as much as that of the hydrocarbons. Inaddition, it is suggested to use zirconium tetrachloride, but the latterexhibits the drawback of being insoluble in a water with a salinityclose to that of seawater.

SUMMARY OF THE INVENTION

The object of this invention eliminates the cited drawbacks andtherefore relates to the use of new compositions of gels for theselective reduction of the production of water in oil- or gas-producingwells.

More specifically, the invention relates to a process for the selectivereduction of water permeability in an oil- and/or gas-producingsubterranean formation in which a composition of aqueous gels isinjected through at least one hydrocarbon-producing well into theformation surrounding the producing well at a suitable flow rate and/orat a suitable pressure and the well is put back into production,characterized in that said composition of aqueous gels comprises asolution of at least one nonionic polysaccharide and at least onecomplex having a base of a water-soluble polyvalent metallic cation andof a complexing organic acid of the cation, able to crosslink saidpolysaccharide, said complex having a concentration, expressed by weightof metal dioxide, of 2 to 100 parts per million of parts of thesolution.

Advantageously, zirconium Zr(IV) or titanium Ti(IV) is used. The organicacid advantageously recommended is an alphahydroxylated acid.

Among the alphahydroxylated organic acids, more particularly analphahydroxycarboxylic acid such as lactic acid or malic acid isselected.

Excellent results in terms of selectivity have been obtained when theacid is lactic acid and the cation is zirconium.

Among the polysaccharides meeting the criteria of the invention, thenonionic polysaccharides such as glucans and in particular scleroglucanand schizophyllan, galactomannane gums such as for example guar gum andin particular its substitution derivatives such as for examplehydroxypropylguar and carboxymethylguar and their mixtures arepreferred.

Among the nonionic polysaccharides of the invention, the preference isgiven to scleroglucan. The latter is a nonionic branchedhomopolysaccharide whose main chain consists of series of patterns ofβ1-3 D-glucose type, substituted every third pattern by a β1-6 D-glucoseunit. Scleroglucan is obtained by fermentation of media containingcarbohydrates initiated by Sclerotium fungi and particularly by a fungusof the Sclerotium Rolfsii type (ATCC I5206).

The zirconium or titanium complexes able to form a gel in aqueous mediumwith scleroglucan are advantageously zirconium or titanium complexeshaving a base of malic or lactic acids, the alphahydroxycarboxylicacid/zirconium or titanium molar ratio preferably being between 0.5 and4 and more particularly between 2 and 4 with lactic acid and 0.5 and 1.5with malic acid. It is possible to cite by way of example the commercialproducts sold by SCPI (Societe des Produits Chimiques Industriels)[Industrial Chemical Products Company] under the names "ZIRCOMPLEX PN"or "ZIRCOMPLEX PA" or further by the ZIRTECH company based inGainesville, Fla. under the name "ZIRTECH LA".

The concentrations by weight of polymer for the obtaining of gelsaccording to the invention generally vary between 150 and 5000 ppm andpreferably between 200 and 2000 ppm.

The concentrations by weight of polyvalent metal complex and preferablyof titanium or zirconium, expressed by weight of metal oxide, varybetween 2 and 100 ppm, advantageously 3 to 95 ppm and preferably between5 and 25 ppm. Concentrations of 100 to 10,000 ppm by weight of metaloxide can even be used, advantageously of 120 to 8000 ppm, andpreferably of I000 to 5000 ppm in certain applications relating to verypermeable or fractured reservoirs where the zones producing water andhydrocarbons are very clearly separated.

By aqueous medium is meant water with all the constituents able to bedissolved there, i.e. the salts, but also other additives necessary foran application such as basic constituents such as soda, or elsesurface-active agents or bactericides.

The composition according to the invention, particularly in the range ofrecommended concentrations (2-100 ppm) can be used to reduce selectivelythe water permeability in a production well without appreciablyaffecting the hydrocarbon permeability. Actually, after having found atthe level of these wells that more water than oil was produced,essentially through the zones of higher permeability, the pumping or theproduction system of these wells is stopped and the compositionaccording to the invention is injected into said wells particularly whenthe range of concentrations of metal oxide corresponds to the formationof a weak gel (2-100 ppm).

After injection of a volume corresponding to a radial extension of thecomposition of 1 to 30 meters from these wells, optionally followed by aclosing time of several days to facilitate on the one hand, theadsorption of the polymer and on the other hand, to make it possible forthe gelling reaction to reach its completion, said wells are put back inproduction and it is found that while producing less water through thezones of high permeability, the production of hydrocarbons through thezones of slight permeability is increased. Putting the wells back intoproduction is generally performed gradually and as much as possible atflow rates and/or pressures equal to or less than those which had beenused for the injection of the aqueous composition.

A first method of putting the formulation in place according to theinvention consists in mixing simultaneously all the constituents of thegel in the well head and in injecting them simultaneously into theformation. The gelling reaction then occurs within the formation to betreated. This method is applied very particularly when it is desired toform a weak gel in a zone of the formation producing water andhydrocarbons simultaneously.

A second method of putting the formulation in place according to theinvention, and corresponding to the use of strong gels in a zone of theformation essentially producing water, consists in making alternatesuccessive injections first of crosslinking solution and then ofpolymer. Thus by successive crosslinkings, an adsorbed multilayer ofpolymer is produced by the crosslinking agent that can go up to theformation of a compact gel with plugging of the water-producing zone.

Preferably the injection of the nonionic polysaccharide mixture and ofcrosslinking complex into the producing well is performed at a flow rateand/or a pressure that is sufficient to enable easy introduction intothe subterranean formation but at a pressure less than the layer limitpressure or breakdown pressure.

By sufficient flow rate and/or pressure is meant a flow rate and/or apressure corresponding to a shear gradient of at least 50 s⁻¹. Theviscosity of the polysaccharide-crosslinking complex mixture at thisgradient is preferably less than 10 mPa.s, for example 1 to 9 mPa.s (1mPa.s=1 cP).

To use the formulation according to the invention and its injection intoa hydrocarbon-producing well, work is performed advantageously at a pHless than 9, the value of the pH depending nevertheless on thetemperature of the reservoir to be treated. For applications at hightemperature, the formulation at a pH close to neutrality is injected.

The examples which follow are intended to illustrate the variousadvantages connected with the use of the formulations according to theinvention. They comprise tests of test specimens making possible theestablishment of the sol/gel phase diagrams as well as tests of placingin porous medium under conditions as close as possible to those existingin the oil-yielding formation.

EXAMPLES Tests of Test Specimens Example 1

Tests in tubes were performed first on raw solutions of powderscleroglucan, "ACTIGUM CS 11 PVE" of the company SANOFI BIOINDUSTRIES,FRANCE, in water containing 50 g/l of NaCl. Different solutions atincreasing concentrations of polymer (of 125 ppm to 3000 ppm) wereprepared, their viscosity measured with an LS 30 viscometer of theCONTRAVES company for a shear gradient of 10 s⁻¹. Increasingconcentrations (5 to 100 ppm) of ZrO₂ of "ZIRTECH LA" (7% by weight ofZrO₂) produced by the company ZIRTECH, USA were added to each of thesepolymer solutions.

The solutions were aged for 5 days in an oven thermoregulated at 30° C.,the viscosities measured again. In table 1, opposite each concentrationof polymer are reproduced the minimum quantities of zirconium complexnecessary to obtain an increase of viscosity of at least 50% of thepolymer solution as a result of a gelling reaction at 30° C. It shouldbe noted that as a result of the addition of the zirconium complex, thepH of the polymer solution gradually changes from 6.5 to 7.3.Furthermore, no gelling or appreciable increase of viscosity is observedfor concentrations of polymer less than I50 ppm (coveringconcentration).

                  TABLE 1                                                         ______________________________________                                        Establishment of the sol/gel diagram for the                                  zirconium lactate-scleroglucan pair (after 5 days at 30° C.)           Scleroglucan   ZrO.sub.2                                                      (ppm)          (ppm)                                                          ______________________________________                                         125           no gelling                                                      250           15                                                              500           10                                                             1000           15                                                             2000           30                                                             3000           50                                                             ______________________________________                                    

Example 2

The tests in tubes of example 1 were renewed except that the differentsamples were aged in an oven thermoregulated at 80° C. and theviscosities were measured at the end of 3 days at this temperature. Theresults of table 2 show that the minimum concentrations of zirconiumlactate are approximately the same at 80° C. and at 30° C. but that thereaction kinetics is notably higher at 80° C. (measurements after 3 daysinstead of 5 days in example 1).

                  TABLE 2                                                         ______________________________________                                        Establishment of the sol/gel diagram for the                                  zirconium lactate-scleroglucan pair (after 3 days at 80° C.)           Scleroglucan   ZrO.sub.2                                                      (ppm)          (ppm)                                                          ______________________________________                                         125           no gelling                                                      250           15                                                              500           15                                                             1000           20                                                             2000           30                                                             3000           50                                                             ______________________________________                                    

Example 3

The tests in tubes of the preceding examples were renewed this time bydispersing hydroxypropylguar powder, "GALACTASOL 476" of the AQUALONcompany, France, in water containing 50 g/l of NaCl. An appreciableincrease of the viscosity corresponding to a gelling of the polymersolution is observed after 3 days at 30° C. and 1 day at 80° C. forsolutions containing respectively 2000 or 3000 ppm of polymer and of"ZIRCOMPLEX PA" of the Societe des Produits Chimiques Industriels (SCPI)titrating 5 ppm of ZrO₂. We note that if in these tests performed at apH of 7, the pH is increased to 9 by addition of a base, an almostinstantaneous gelling of the polymer solutions is obtained even atambient temperature.

Tests in Porous Medium Examples 4 to 8

To test according to the invention the effectiveness of the weakgel-based formulations to reduce water permeability without affectingthe hydrocarbon permeability, there was applied to various porousmediums the experimental procedure described in the communication of A.Zaitoun and N. Kohler to the Society of Petroleum Engineers under thereference SPE 18085 of October 1988 and comprising the follow stages:

1. Saturation of the porous medium with brine and determination ofinitial water permeability k_(wi).

2. Injection of oil up to irreducible water saturation S_(wi).

3. Injection of brine up to irreducible oil saturation S_(or).

4. Operations 2 and 3 are repeated until the extreme values of relativepermeabilities k_(rw) and k_(ro) are reproducible.

5. At the irreducible oil saturation, injection of the polymer alone(reference) or the polymer+crosslinking agent. Stopping of thecirculation of the fluids to make it possible for the gelling reactionto reach its completion.

6. Injection of brine until all of the nonadsorbed polymer is displaced.It is verified that the viscosity of the effluent corresponds to that ofthe brine. The reduction curve of water permeability R_(kw) is plottedas a function of the flow rate or shear gradient γ.

7. Injection of oil up to irreducible water saturation S_(wi).Determination of the new value of k_(ro) at high flow rate (Welgemethod).

8. Injection of water up to S_(or) and determination of the new value ofk_(rw) at high flow rate.

Shear gradient γ in porous medium is calculated as follows: ##EQU1##where v is the surface velocity calculated by: ##EQU2## where q is theinjection flow rate, S the surface of the input face of the porousmedium, φ the porosity r the average radius of the pores calculated by:##EQU3## where k is the initial water permeability of the porous medium.The reduction of brine permeability R_(kw) is a measurement of theapparent viscosity of the brine circulating in the porous medium afterputting the polymer or the gel in place.

The measurements of relative permeabilities k_(rw) and k_(ro), made atthe same saturation condition S_(w) and for the same flow rate q,correspond to load losses due to the circulation of the fluids, water oroil, before and after putting the polymer or the gel in place.

Example 4

The first experiment of putting the formulation according to theinvention in place was performed on a Vosges sandstone core sampleinserted in a Hassler cell, the whole being put in an oven at 95° C.Pressure sensors make it possible to measure the load losses at theboundaries of the core and a positive-displacement pump makes itpossible to inject the fluids at a constant flow rate.

Initial water permeability k_(wi) (140 g/l of total salinity) was foundequal to 1.5 D. Successively, the injection of oil (μ=1.97 cP at 95° C.)was performed up to its irreducible water saturation and the relativeoil permeability (k_(ro) =0.92, S_(wi) =0.34 and k_(ro) =0.28, S_(w)=0.48) was measured, then the water injection was performed up toirreducible oil saturation and the relative water permeability (k_(rw)=0.05, S_(or) =0.46) was measured.

Then the injection was performed of a solution at 1.5 g/l of powderscleroglucan, "ACTIGUM CS 11 PVE" of the company SANOFI BIOINDUSTRIES,dispersed in salt water at the flow rate q=20 cm³ /hr. The adsorption ofthe polymer was found equal to 120 μg/g.

The injection of brine was then performed to displace the nonadsorbedpolymer and the reduction of water permeability R_(KW1) was thenmeasured at different water injection flow rates (table 3).

The core sample was then saturated using a mixture of scleroglucan(Cp=1.5 g/l) and zirconium complex, "ZIRTECH LA" (C_(ZrO2) =30 ppm), amixture which gives rise to a tube gelling reaction at a temperature of95° C., and all circulation of fluid was stopped for 16 hours. Thesurplus of the mixture which was not absorbed or which had not reactedwas displaced by water injection and a new measurement of the reductionof water permeability was performed at various flow rates R_(KW2) (table3).

                  TABLE 3                                                         ______________________________________                                        Vosges sandstone at 95 and 120° C.                                     Reductions of water permeability after polymer alone (R.sub.KW1)              and after polymer + zirconium lactate (R.sub.KW2)                                                     R.sub.KW1                                                                              R.sub.KW2                                    Flow rate Shear Gradient                                                                              polymer  polymer +                                    q(cm.sup.3 /hr)                                                                         γ(s.sup.-1)                                                                           alone    Zr complex                                   ______________________________________                                         4        12.4          5.00     64.3                                         10        31            3.80     42.4                                         30        93            3.13     24.9                                         50        155           2.76     12.1                                         ______________________________________                                    

It is found that the values of reduction of water permeability afterwater injection of the mixture according to the invention are largelygreater than those measured after injection of polymer alone.

Moreover, it is possible to show by using the Welge method that therelative oil permeability (k_(ro) =0.24, S_(w) =0.48) is not affected bythe presence of the weak gel whereas the relative water permeability(k_(rw) =0.011, S_(w) =0.54) is greatly reduced relative to the initialpermeability (k_(rw) =0.05 at the same water saturation value).

The formulation according to the invention therefore greatly reduces thewater permeability without appreciably affecting the oil permeability.The system therefore is selective.

The temperature of the oven was then brought to 120° C. to test thestability of the formulation at this temperature in porous medium.Synthetic seawater, carefully deoxygenated by addition of 100 ppm ofsodium sulfite, was continuously injected under a nitrogen atmosphere ata flow rate of 1 ml/hr in the porous medium and the pressures measureddaily for 14 days at 120° C. It was found that the value of permeabilityreduction measured at this flow rate (R_(k) =103) proved particularlystable during this experiment (final value (R_(k) =95).

The formulation according to the invention therefore makes it possibleto reduce the water permeability even at this temperature for ratherextended times.

Example 5

The preceding experiment was renewed this time by using a core sample ofsand of "ENTRAIGUES EN 38" reconstituted in a Hastelloy stainless steelcell, the whole placed in an oven at 80° C.

The relative water permeability (reconstituted seawater) of the coresample was found equal to 2.32 D. Successively, the injection of oil(μ=2.43 cP at 80° C.) up to irreducible water saturation was performedand, as before, the relative oil permeability was measured for 2 watersaturation values (k_(ro) =0.615, S_(w) =0.30 and k_(ro) =0.22, S_(w)=0.43). As a result of the water injection, the initial values ofrelative water permeability were determined (k_(rw) =0.30, S_(w) =0.79).

The injection of a solution of scleroglucan (Cp=1.5 g/l) in seawatergives rise to an irreversible adsorption of the latter equal to 80 μg/g.

As a result of the water injection and the total displacement of thenonadsorbed polymer, the values of permeability reduction R_(KW1) oftable 4 are observed.

                  TABLE 4                                                         ______________________________________                                        Entraigues sand at 80° C.:                                             Reductions of water permeability after polymer alone (R.sub.KW1)              and after polymer + zirconium lactate (R.sub.KW2)                                                     R.sub.KW1                                                                              R.sub.KW2                                    Flow rate Shear Gradient                                                                              polymer  polymer +                                    q(cm.sup.3 /hr)                                                                         γ(s.sup.-1)                                                                           alone    Zr complex                                   ______________________________________                                        10        10.2          4.39     179.8                                        20        20.4          2.74     118.0                                        30        30.6          2.30     95.8                                         50        51            1.97     74.1                                         100       102           1.75     57.7                                         200       204           1.48     45.0                                         ______________________________________                                    

Then the injection was performed of a formulation according to theinvention containing 1.5 g/l of powder scleroglucan and 25 ppm ofzirconium complex "ZIRCOMPLEX PA" of the SCPI titrating 7.3% by weightof ZrO₂, a formulation able to form a weak gel at 80° C., and allcirculation of fluid was stopped for 16 hours.

Then, as before, the injection of water and the measurement of thereduction of water permeability at different flow rates R_(KW2) (table4) were performed. Here also the values obtained are largely greaterthan those resulting from the adsorption of the polymer alone.

The selectivity of the system is proved by comparing the relative oiland water permeabilities before and after putting the weak gel in place(Welge method). The relative water permeability k_(rw) thus goes from aninitial value equal to 0.30 to a final value equal to 0.012 for the samesaturation condition S_(w) =0.79. The relative oil permeability k_(ro)goes from an initial value equal to 0.22, S_(w) =0.43 to a final valuevery close to 0.17 at the same saturation condition.

The selectivity of the formulation according to the invention is thusagain demonstrated.

Example 6

The preceding experiments were conducted again, this time using a porousmedium consisting of St. Waast les Mello limestone inserted in a Hasslercell and put in the oven at 80° C.

The initial seawater permeability was found equal to 93I mD. The porousmedium was put in residual oil (μ=2.40 cP) and the seawater permeabilityin the presence of residual oil k_(SOR) was found equal to 157 mD.

As before, the injection of a solution of powder scleroglucan (Cp=1500ppm) that is used as a reference was performed, then the injection of amixture of polymer (Cp=1500 ppm) and "ZIRTECH LA" (C_(ZrO2) =24 ppm) wasperformed. After a stop of 19 hours at 80° C., the values of reductionof water permeability were measured.

Table 5 gathers the results of reduction of water permeability afterrespective putting in place of the polymer alone, then the formulationaccording to the invention.

An aging test was also performed by injecting seawater at a low flowrate for 13 days at 80° C. and by performing a daily measurement of thereduction of water permeability. Table 5 shows that the values obtainedare perfectly stable over time (R_(KW3)).

In the same way as before, it is demonstrated that the weak gel isselective in carbonated medium, greatly reducing the water permeabilitywithout changing the oil permeability to a great extent.

                  TABLE 5                                                         ______________________________________                                        St. Waast les Mello limestone at 80° C.                                Reductions of water permeability after polymer alone (R.sub.KW1)              and after polymer + zirconium lactate (R.sub.KW2) and                         after (R.sub.KW3) aging                                                                           R.sub.KW1                                                 Flow rate                                                                             Shear Gradient                                                                            polymer  R.sub.KW2                                                                             R.sub.KW3                                q(cm.sup.3 /hr)                                                                       γ(s.sup.-1)                                                                         alone    polymer + Zr complex                             ______________________________________                                         2      4.9         11.80    64.6    103.50                                   10      24.5        4.48     23.54   24.95                                    20      49          3.98     16.66   16.67                                    50      122.5       3.30     11.52   12.53                                    100     245         3.01     9.50    10.3                                     ______________________________________                                    

Example 7

In a core sample of Entraigues sand put in an oven at 50° C. and whosesynthetic seawater permeability (30 g/l of NaCl and 3 g/l of CaCl₂ ·2H₂O) was found equal to 4.8 D, the injection was performed of a solutionat 2500 ppm of hydroxypropylguar, "GALACTOSOL 476" of the AQUALONCompany, in seawater at a flow rate of 20 ml/hr. This injection ofpolymer was followed by injection of seawater to displace thenonadsorbed excess polymer and a reduction of water permeability of 1.8was measured that was practically independent of the water injectionflow rate. In the same core sample, there was then performed at the sameflow rate of 20 ml/hr the injection of a mixture in synthetic seawatercontaining 2500 ppm of hydroxypropylguar, 10 ppm of "ZIRCOMPLEX PN"(titrating 9.9% by weight of ZrO₂) and 100 ppm of citric acid and aftera stopping of circulation for one night at 50° C. there was performed asbefore the injection of synthetic seawater to displace the excesspolymer. The final reduction of seawater permeability was found equal to290 at this same flow rate of 20 ml/hr.

Example 8

In a core sample of Entraigues sand at 80° C. saturated in syntheticseawater and whose permeability was found equal to 4.3 D, the alternateinjection was performed of 1.5 porous volume of a solution of"ZIRCOMPLEX PN" (titrating 1000 ppm of ZrO₂) in seawater at a flow rateof 50 ml/hr followed by 1.5 porous volume of a solution of scleroglucan(Cp=500 ppm) in seawater. All circulation was stopped for 24 hours toenable the gelling reaction to take place. Then the injection ofseawater was performed at the same flow rate of 50 ml/hr and it wasfound that the load losses at the boundaries of the porous medium weregreatly increased and that the seawater permeability had become veryslight. By calculation, a reduction of water permeability greater than1000 is obtained.

The alternate injection of crosslinking agent and of polymerconsiderably reduces the water permeability as a result of the formationof a strong gel. An oil injection test in this porous medium also givesrise to very high load losses thus showing that the formulation used isno longer selective and blocks both the passage of water and that ofoil.

We claim:
 1. A process for the selective reduction of water permeabilityin a subterranean formation, and producing oil and/or gas therefrom,said process comprising injecting a composition of aqueous gels at nearneutral pH through at least one hydrocarbon-producing well into theformation surrounding a producing well at a flow rate and/or at apressure corresponding to a shear gradient of at least 50 s⁻¹ and thewell is put back into production, wherein said composition of aqueousgels consists essentially of a solution of at least one nonionicpolysaccharide, which is a glucan, galactomannane gum or a mixturethereof, and at least one complex of a water-soluble polyvalent metalliccation which is zirconium or titanium and an alphahydroxyl complexingorganic acid of the cation, able to crosslink said polysaccharide, saidcomplex having a concentration, expressed by weight of metal dioxide, of2 to 100 parts per million of parts of the solution, whereby thepermeability of the formation of water is reduced without significantlyreducing the permeability to hydrocarbons.
 2. A process according toclaim 1, wherein the nonionic polysaccharide is scleroglucan.
 3. Aprocess according to claim 1, wherein the complexing organic acid of thecation is lactic acid or malic acid.
 4. A process according to claim 1,wherein the concentration of the complex is 3 to 95 ppm, so that thewater permeability is greatly reduced without the oil permeability beingappreciably affected.
 5. A process according to claim 1, wherein theconcentration of the complex is 100 to 10,000 ppm, when the subterraneanformation is fractured or very permeable.
 6. A process according toclaim 1, wherein the concentration of the complex is 5 to 25 ppm.
 7. Aprocess according to claim 1, wherein the water of the subterraneanformation is at a temperature of 70° to 130° C.
 8. A process accordingto claim 1, wherein the viscosity of the composition is less than 10mPa.s at said shear gradient.
 9. A process according to claim 1, whereinthe complexing organic acid of the cation is lactic acid.
 10. A processaccording to claim 9, wherein the lactic acid/zirconium molar ratio isbetween 2 to
 4. 11. A process according to claim 1, wherein thecomplexing organic acid of the cation is malic acid.
 12. A processaccording to claim 14, wherein the malic acid/titanium molar ratio isbetween 0.5 and 1.5.
 13. A process according to claim 1, wherein theconcentration of the complex is 3 to 95 ppm so that the waterpermeability is greatly reduced without the oil permeability beingappreciably affected.
 14. A process according to claim 1, wherein theconcentration of the complex is 120 to 8000 ppm when the subterraneanformation is fractured or very permeable.
 15. A process for theselective reduction of water permeability in a subterranean formation,and producing oil and/or gas therefrom, said process comprisinginjecting a composition of aqueous gels at near neutral pH through atleast one hydrocarbon-producing well into the formation surrounding aproducing well at a flow rate and/or at a pressure corresponding to ashear gradient of at least 50 s⁻¹ and the well is put back intoproduction, wherein said composition of aqueous gels consistsessentially of a solution of at least one nonionic polysaccharide, whichis a glucan, and at least one complex of a water-soluble polyvalentmetallic cation which is zirconium or titanium and an alphahydroxylcomplexing organic acid of the cation, able to crosslink saidpolysaccharide, said complex having a concentration, expressed by weightof metal dioxide, of 2 to 100 parts per million of parts of thesolution, whereby the permeability of the formation of water is reducedwithout significantly reducing the permeability to hydrocarbons.
 16. Aprocess for the selective reduction of water permeability in asubterranean formation, and producing oil and/or gas therefrom, saidprocess comprising injecting a composition of aqueous gels at nearneutral pH through at least one hydrocarbon-producing well into theformation surrounding a producing well at a flow rate and/or at apressure corresponding to a shear gradient of at least 50 s⁻¹ and thewell is put back into production, wherein said composition of aqueousgels consists essentially of a solution of at least one nonionicpolysaccharide, which is a glucan, galactomannane gum or a mixturethereof, and at least one complex of a water-soluble polyvalent metalliccation which is zirconium or titanium and an alphahydroxyl complexingorganic acid of the cation, able to crosslink said polysaccharide, saidcomplex having a concentration, expressed by weight of metal dioxide, of2 to 100 parts per million of parts of the solution, whereby thepermeability of the formation to water is reduced without significantlyreducing the permeability to hydrocarbons,with the proviso that thecomposition of aqueous gels does not contain amine compounds.